Different parts of the central nervous system exhibit differing degrees of sensitivity to the effects of oxygen-lack. It is hypothesized that this differential sensitivity is the result of local differences in the cerebrovascular response to hypoxia. To test this hypothesis neural function, tissue oxygen levels, and local blood flow will be monitored in barbiturate anesthetized cats subjected to graded episodes of hypoxic hypoxia. Oxygen-sensing microelectrodes at lateral geniculate nucleus (LGN) and visual cortex (VC) will monitor visual evoked responses and local PO2. A technique using thermal energy as a tracer for monitoring local blood flow will be developed to provide information about changes in regional cerebral blood flow. These three physiological parameters will be monitored before, during and after episodes of hypoxic hypoxia. The data will be analyzed to (1) correlate failure of the neural response with the existing local PO2 and rCBF and (2) determine whether function is maintained at brainstem sites (LGN) longer than at the level of the cortex (VC) by an increased (absolute or relative) blood flow at that level.